Add multiple b0 slices to medotsu4D. Fix other minor review comments.

medotsu4D is now able to take a single int OR a tuple of b0Slice indexes.
If multiple slice indexes are provided, the mean value of all slices is used.
Adjust reconst_dti and reconst_csa to use the new medotsu4D multislice.
Various code format modifs from review
Binary thresholding now returns dtype=bool numpy array.
Refactored bbox computing, add python3 compat.
Update the brain_extraction_dwi example to a standard example.
Remove print
Various fixes from mask.py reviews.

Simplyfied masking code.
Fixed original data being modified with medotsu.
Adjust examples to fixed medotsu behavior(no inplace modif)
Crop is now generalised for n dimensions.
Refactored medotsu to work with 3D and 4D cases.

Removed medotsu4D
Adjust medotsu comments
Fix bug modifing input volume on autocrop
Adjust examples to new medotsu signature